1. Field of the Invention
The present invention relates generally to using radio signals to estimate separation distances between transmitting and receiving radios, and more particularly, to a system and method of estimating a separation distance between an impulse radio transmitter and an impulse radio receiver based on a signal strength of an impulse signal at the receiver.
2. Related Art
A known time-based system for estimating a separation distance between a transmitter and a receiver translates a transit time of a radio signal transmitted between the transmitter and the receiver into a separation distance estimate. Such a system disadvantageously requires complex transceiver circuitry and/or techniques for time-synchronizing the receiver to the transmitter, and for measuring signal time-of-arrival at the receiver.
A known amplitude-based system includes a transmitter for transmitting a radio signal, such as a continuous wave signal having a relatively narrow band frequency characteristic, and a receiver for receiving the radio signal and for estimating a separation distance between the transmitter and the receiver based on a signal strength or amplitude measurement of the narrow band signal at the receiver. Although less complex and generally less expensive than the time-based system, the narrow band, amplitude-based system disadvantageously suffers from multipath fading processes. An example of such multipath effects is called Rayleigh fading, in which signals from many delayed reflections combine and destructively interfere with one another at an antenna of the receiver. This is especially true of mobile urban environments and within edifices where multipath effects may be so extreme as to cause 6 decibels (dB) or more root-means-square signal strength deviations with more than 30 dB or more signal strength nulls at the receiver, resulting in thirty-to-one (30:1) distance measurement errors. Such large signal strength fluctuations as a function of receiver position lead to inaccurate separation distance estimates and render known amplitude-based systems unusable. Efforts to overcome multipath fading include diversity techniques such as using multiple frequencies and multiple antennas, as well as using spread spectrum coded waveforms. However, such efforts have had only limited success.
Therefore, there exists a need to resist or mitigate the effects of multipath fading in a system for estimating a separation distance between a mobile radio transmitter and a mobile radio receiver based on a signal strength measurement at the receiver.